Fire detector with red and blue responsive photocells and regenerative feedback

ABSTRACT

A fire detector in which a pair of photo-resistive cells which respond differently to radiation from a fire and a pair of fixed resistors are connected in voltage divider circuits across a common source, with an alarm actuating device connected between the junction of the cells and the junction of the fixed resistors. The alarm actuating device responds to a reversal of relative polarity of said junctions to actuate the alarm. In one embodiment of the invention the portion of the alarm actuation means connected between the junctions is the base-emitter circuit of the first transistor of a transistor amplifier. In a modified form of the invention, on actuation of the alarm, additional resistance is connected into the circuit to shift the drop-out point of the alarm actuator to a desired point.

FIRE DETECTOR WITH RED AND BLUE RESPONSIVE PHOTQCELLS AND REGENERATIVEFEEDBACK Continuation of Ser. No. 823,584, May 12, 1969, abandoned.

U.S. Cl. ..250/83.3 H, 340/233, 340/237 S Int. Cl. ..G08b 21/00 Field ofSearch ..340/233, 237 S; 250/833 IR;

References Cited UNITED STATES PATENTS 12/1942 Lord ..328/2 l/l939Berges .328/2 12/1947 Gieseke ..328/2 12/1965 Vasel et al ..340/2 28Primary Examiner-James W. Lawrence Assistant Examinerl-Iarold A. DixonAttorney-Robert E. Ross [57] ABSTRACT A fire detector in which a pair ofphoto-resistive cells which respond differently to radiation from a fireand a pair of fixed resistors are connected in voltage divider circuitsacross a common source, with an alarm actuating device connected betweenthe junction of the cells and the junction of the fixed resistors. Thealarm actuating device responds to a reversal of relative polarity ofsaid junctions to actuate the alarm. in one embodiment of the inventionthe portion of the alarm actuation means connected between the junctionsis the base-emitter circuit of the first transistor of a transistoramplifier. In a modified form of the invention, on actuation of thealarm, additional resistance is connected into the circuit to shift thedrop-out point of the alarm actuator to a desired point.

2 Claims, 1 Drawing Figure PATENTEDAPR3 ma I 11 van for W/V/iam FDa/z67' 211 5 72/5 Altar/26y I l l l l I l l I l I 5 l l I FIRE DETECTORWITH RED AND BLUE RESPONSIVE PHOTOCELLS AND REGENERATIVE FEEDBACK Thisapplication is a continuation of application Ser. No. 823,584 filed May12, 1969, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to optical fire detection apparatus in which the opticaldetection means comprises a pair of photo-resistive cells which responddifferently to radiation from a fire, the cells being connected as avoltage divider, with the change in voltage at the junction of the cellsbeing utilized to actuate an alarm.

2. Description of the Prior Art Optical fire detection systems of thetype utilizing photo-resistive cells connected as a voltage dividercustomarily utilize the rise in voltage at the junction of the cells asthe input to a transistor amplifier, with the output of the amplifierbeing connected to a relay controllingthe operation of an alarm device.As the voltage at the junction between the cells increases, the outputof the amplifier increases. When a predetermined voltage at the junctionis reached, the output of the amplifier is sufficient to actuate therelay to energize the alarm. Such a system is illustrated in U.S. Pat.No.

3,122,638, issued Feb. 25, 1964. Another form of photo-resistive cellassembly for use in such a system is illustrated in U.S. Pat. No.3,188,593, issued Although fire detectors of this type have achievedconsiderable commercial success, one practical difficulty encountered inmanufacturing such a system is the fact that the relay used mustmeet'exceptionally high quality standards. The reason for this is thefact that as the radiation in the specified frequency band increases,the voltage at the junction of the detector cells gradually increases,and the current in the relay circuit gradually increases. To insure thatthe relay armature will pull in at a predetermined level of radiation, arelay is required that has a pull-in valve that lies withinpredetermined narrow limits. In actual commercial construction practice,to achieve consistent performance, it has been found necessary to pay aconsiderable premium for suitable relays and also necessary to test theactual pullin current of each individual relay before it is assembledinto a detector system.

SUMMARY OF THE INVENTION This invention disclosed herein relates to afire detector in which a pair of photo-resistive cells having differentresponse characteristics to radiation from a fire and a pair of fixedresistors are connected in voltage divider circuits across a commonsource, with an alarm actuating device connected between the junction ofthe cells and'the junction of the fixed resistors. The alarm actuatingdevice responds to a reversal of relative polarity of said junctions toactuatethe alarm. In one embodiment of the invention the portion of thealarm actuating means connected between the junctions is thebase-emitter circuit of the first stage of a transistor amplifier. In amodified form of the invention, on actuation of the alarm, additionalresistance is connected into the circuit to shift the dropout point ofthe alarm actuator to a desired point.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE of the drawing is aschematic diagram of a fire detector embodying the features of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawing, thereis illustrated an optical fire detector system comprising a detectorportion 10, and amplifier portion 12, for actuating an alarm relay 14 toenergize an alarm A when the detector portion 10 receives radiation froma fire, as will be described hereinafter.

The detector portion 10 may be of a type disclosed in either of theabove-identified patents and comprises a photo-resistive cell PRprimarily responsive to radiation in the red to infra-red band, and aphoto-responsive cell PB which is primarily responsive to radiation inthe blue band. The cells PR and PB are connected in series through acommon junction J1 across a DC source S. The effect of fire radiation onthe detector cells is to cause an increase in voltage at the junction J1in a manner and for a purpose to be described hereinafter.

The amplifier 12 comprises a first stage having a transistor T1 with itscollector-emitter circuit connected across the DC source, and the baseconnected to the junction J1 of the detector cells, and a second stagehaving a transistor T2 with its collector-emitter circuit connected inseries with the coil of the alarm relay 14 across the DC source with thebase thereof being connected to the collector of transistor Tll.

The relay 14 has a first set of contacts 14 X 1 which, in thede-energized condition of the relay, are open, and which close onenergization of the relay coil to complete a circuit to the alarm 16.

The relay 14 has a second set of contacts 14 X 2 which, in thede-energized condition of the relay are open and whichclose to completea circuit between the emitter of transistor T1 and the negative pole ofthe DC source through a resistor R1 for a purpose to appear hereinafter.

To provide a reference voltage to control the baseemitter bias appliedto the transistor T1, and to therefor control the conduction in thecollector-emitter circuit thereof, a pair of fixed resistors R2 and R3are connected in series across the DC source though a junction J2, withsaid junction being connected to the emitter of the transistor T1.

Conduction through the transistor T1, and therefore conduction throughtransistor T2 and the alarm relay depends on the voltage between thebase and emitter of transistor T1. During normal standby operation ofthe device, when the detector is not exposed to radiation from a firebut is exposed to ambient light such as indirect sunlight, incandescent,or fluorescent light, since the blue component of such light is greaterthan the infra-red component, the cell PR will have a higher resistancethan cell PB. The ratio of the resistance of PR to the resistance of PBis therefore greater than the resistance ratio of R2/R3 and the junctionJ l is therefore negative in relation to the junction J2. The base oftransistor T1 is therefore negatively biased in relation to the emitterand conduction cannot occur in the emitter-collector circuit thereof, sothat the coil of relay 14 remains unenergized.

However, when the detector is exposed to a fire of sufficient intensity,the infra-red radiation therefrom causes the resistance of cell PR todrop to a value such that the resistance ratio of PR/PB becomes lessthan the resistance ration of R2/R3. The junction J1 thereupon becomespositive in relation to the junction J2, and the base of transistor T1therefore becomes positive in relation to the emitter, allowingconduction to occur in the collector-emitter circuit. Transistor T2 isthereby biased into conduction, energizing the coil of relay l4 andactuating the alarmin a specific embodiment of the invention, theresistors R2 and R3 may each have a resistance of 1000 ohms. Underconditions of normal ambient light the cell PR may have a resistance ofperhaps 1 million ohms or more, and the cell PB a resistance of perhapsmillion ohms. Junction J1 is therefore negative in v relation tojunction J2.

However, when infra-red radiation from a fire is received by thephoto-cells, the resistance of the cell PR, may drop to 5000 ohms,whereas the resistance of cell PB drops only to 25,000 ohms, so thatjunction J1 becomes positive in relation to J2 with the resultpreviously described.

Since many fires, in their initial stages, are erratic in their outputof infra-red radiation, it has been found desirable to provide means foradjusting the circuit, once there has been energization of the relay 14so that less infra-red radiation will be required thereafter to hold therelay in the energized condition than was required to initially energizeit.

For this purpose, the energization of the relay 14 also closes contacts14 X 2 which places resistor R1 in parallel with resistor R3 and therebylowers the voltage at J2. Since the voltage at J1 is unchanged a greaterforward bias voltage is therefore applied to the transistor T1. When amomentary decrease of infra-red radiation occurs, the relay 14 willremain energized, so long as sufficient radiation is received by thedetector to maintain the voltage at the junction J1 higher than thevoltage existing at J2 with the additional resistors R1 in the circuit.

If the detector system is used in conjunction with an extinguishingsystem, the use of the above described means of lowering the drop-outpoint of the relay will insure that the alarm will continue to beenergized until the tire radiation has dropped to an intensity less thanthat which initiated the alarm.

Although in the illustrated embodiment, the dropout point of the relayis changed by reducing the resistance between junction J2 and thenegative side of the supply voltage an equivalent effect can beobtainedis intended that the above descriptions of a specific embodimentof the invention be interpreted in an illustrative and not a limitingsense.

Having thus described my invention, what I claim as new and desire tosecure by Letters Patent of the United States is:

1. A fire detector, comprising a transistor amplifier,

alarm actuating means, and a power source, said amplifier triggeringsaid alarm actuating means when the input transistor thereof conducts inthe collectoremitter path, the emitter of said input transistor beingconnected to opposite poles of the power source by a pair of resistorsto provide a predetermined voltage at the emitter, the base of saidinput transistor being connected to opposite sides of the power sourceby a pair of photo-cells, said cells being so associated that they viewthe same space, one of said cells being responsive by a decrease inresistance to light in the red to infrared band, the other cell beingresponsive by a decrease in resistance to light in the blue band,whereby a change from ordinary ambient light to radiation from a firewill cause a change in voltage at said base in relation to the voltageat the emitter, the value of the resistors connected to the emitterbeing such that during normal standby conditions, a reverse bias isapplied to the base, and when the voltage at the base changes due to thechange in resistance of the photo-cells due to radiation from a fire, aforward bias is applied to the base to allow conduction in the emittercollector path to trigger said alarm actuating device, and meansresponsive to energization of the alarm actuating means for changing thevalue of the resistance between one pole of the power source and theemitter of said input transistor in such a manner as to increase theforward bias on the base, whereby the level of radiation from a fire atwhich the alarm actuating means is deenergized is less than the level atwhich it was energized.

2. A fire detector, comprising a pair of photo-cells so associated thatthey view the same space, one of said cells being responsive by adecrease in resistance to light in the red to infra-red band, the othercell being responsive by a decrease in resistance to light in the blueband, said cells being connected in series across a power source througha first junction, a pair of resistors connected in series across saidpower source through a second junction, whereby radiation from a firereceived by said photo-cells will cause a change in voltage between saidjunctions, means responsive to said change in voltage to actuate analarm energizing means, and means responsive to the actuation of saidalarm energizing means to further change the voltage between saidjunctions in a direction such that on decrease of the amount ofradiation from a fire, the alarm will continue to be energized at alevel of fire radiation less than the level of radiation at which it wasenergized.

1. A fire detector, comprising a transistor amplifier, alarm actuatingmeans, and a power source, said amplifier triggering said alarmactuating means when the input transistor thereof conducts in thecollector-emitter path, the emitter of said input transistor beingconnected to opposite poles of the power source by a pair of resistorsto provide a predetermined voltage at the emitter, the base of saidinput transistor being connected to opposite sides of the power sourceby a pair of photo-cells, said cells being so associated that they viewthe same space, one of said cells being responsive by a decrease inresistance to light in the red to infra-red band, the other cell beingresponsive by a decrease in resistance to light in the blue band,whereby a change from ordinary ambient light to raDiation from a firewill cause a change in voltage at said base in relation to the voltageat the emitter, the value of the resistors connected to the emitterbeing such that during normal standby conditions, a reverse bias isapplied to the base, and when the voltage at the base changes due to thechange in resistance of the photo-cells due to radiation from a fire, aforward bias is applied to the base to allow conduction in the emittercollector path to trigger said alarm actuating device, and meansresponsive to energization of the alarm actuating means for changing thevalue of the resistance between one pole of the power source and theemitter of said input transistor in such a manner as to increase theforward bias on the base, whereby the level of radiation from a fire atwhich the alarm actuating means is de-energized is less than the levelat which it was energized.
 2. A fire detector, comprising a pair ofphoto-cells so associated that they view the same space, one of saidcells being responsive by a decrease in resistance to light in the redto infra-red band, the other cell being responsive by a decrease inresistance to light in the blue band, said cells being connected inseries across a power source through a first junction, a pair ofresistors connected in series across said power source through a secondjunction, whereby radiation from a fire received by said photo-cellswill cause a change in voltage between said junctions, means responsiveto said change in voltage to actuate an alarm energizing means, andmeans responsive to the actuation of said alarm energizing means tofurther change the voltage between said junctions in a direction suchthat on decrease of the amount of radiation from a fire, the alarm willcontinue to be energized at a level of fire radiation less than thelevel of radiation at which it was energized.